Radiation imagers are widely used in medical diagnostic tests, industrial non-destructive evaluation, and scientific research. Large area radiation detectors, i.e. having a detection area exposed to the incident radiation of more than about 100 square centimeters, have conventionally comprised intensifier screens or stimulated phosphors.
In an intensifier screen x-ray detector, for example, the incident radiation strikes a phosphor layer which converts the x-ray radiation into a light image. In a typical arrangement, an optical lens focused on the phosphor screen couples the light output to a television camera which generates an associated electrical signal that can be processed and displayed. A major drawback of the phosphor intensifier screen device is that it lacks the sensitivity and acuity demanded in medical, scientific, and industrial testing and analysis, and is cumbersome because of the amount of television equipment required to produce the image.
A stimulated phosphor device is a flat panel x-ray sensor in which the incident x-ray flux stimulates electrons in the phosphor layer, which are then stored in electron traps at the higher energy state. The phosphor panel is read or analyzed by a laser beam sweeping over the panel, detrapping the electrons and causing a radiative recombination in which light of a different wavelength than the laser beam is emitted. A photodetector sensitive to the emitted light collects the emitted light and generates an electrical signal which is processed to produce an image on conventional display equipment. This type of detector does not allow for real-time presentation of x-ray images, and similarly has sensitivity limitations which restrict its use in many testing and analysis applications.
Panel radiation converters have been proposed which use radiation absorbent scintillators coupled to photodetectors such as photodiodes. For example, a solid state radiation imager is disclosed by S. E. Derenzo in U.S. Pat. No. 4,672,207, issued Jun. 9, 1987. In the Derenzo device, radiation incident on the detector strikes a scintillator which is divided into rows and columns. An array of photodetectors underlies the scintillator and generates electrical signals dependent on the light produced in the scintillator as the radiation is absorbed. Derenzo discloses the use of silicon avalanche photodiodes as photodetectors. There are, however, several drawbacks to the Derenzo device that would be noted by a user, such as the high noise and low sensitivity of the data collection and amplification circuits disclosed in the patent and the difficulty of adapting the Derenzo device to a large area array using the crystalline silicon photodiodes disclosed in the patent. Another approach to a high resolution camera for radiation imaging is disclosed in the copending patent application entitled "Photodetector Scintillator Radiation Imager," Ser. No. 07/746,847, filed Aug. 19, 1991, now U.S. Pat. No. 5,144,141, of H. Rougeot, et al., and assigned to the assignee of the present invention.
It is difficult in a mass production process to uniformly fabricate the large number of avalanche photodiodes (APDs) necessary for a large area array when the APDs are made of silicon, gallium arsenide, or other monocrystalline photosensitive materials. Such materials are not readily deposited and patterned in large arrays with traditional semiconductor fabrication techniques. It is desirable to use an amorphous material that exhibits avalanche gain multiplication, i.e. a gain of 10.times. or more, and which can readily be fabricated in a large area array in devices requiring large gain.
Avalanche multiplication has been observed in amorphous selenium, as reported by K. Tsuiji et al. in "Avalanche Multiplication in Amorphous Selenium," Extended Abstracts of the 19th Conference on Solid State Devices and Materials, Japan Society of Applied Physics August 1987, pp. 91-94. Amorphous selenium, when polarized to a high voltage (i.e., greater than about 10.sup.6 volts/cm) to cause avalanche multiplication, exhibits high ionization rates with comparatively low noise. Although amorphous selenium has been used as a photosensitive material in photodiodes for facsimile machines, photocopiers, and the like, in such uses the biasing voltage is relatively low (about 100 volts or less) and the avalanche effect is not used. Small size, i.e. between 1 cm.sup.2 and 5 cm.sup.2, amorphous selenium arrays have been proposed for use in in low light conditions in certain television camera devices in which it is thought the avalanche effect could be used.
It is therefore an object of this invention to provide a photosensor for a high sensitivity large area radiation detection array that is readily fabricated and that exhibits avalanche multiplication when in use.
It is a further object of this invention to provide a radiation detector capable of operating at the high voltages necessary to obtain avalanche characteristics in the photosensor while protecting associated data read and amplification circuitry.
It is a still further object of this invention to provide a high gain, low noise photosensitive radiation detection array for real time detection and display of x-ray radiation having low energy levels.